Our kids really need to know that. Like now. Our younger teachers do as well. In your next legal training on anything related to social media, you should mention this. I teach a pre-service law course starting next week; I am definitely going to mention this in there.
I'm not a user of snapchat myself, but I know that many of our students are heavy users somewhat on the premise that whatever they send will go away quickly. Well, no so much.